1. Field of the Invention
The present invention relates to a television signal transmission system, and more particularly to a VSB demodulator of a digital television signal receiver employing the VSB demodulation system.
2. Related Art of the Invention
A conventional VSB demodulator is disclosed, for example, in Japanese Laid-open Patent No. 7-326951.
FIG. 16 shows a constitution of this conventional VSB demodulator. In this demodulator, an RF signal of a received digital VSB modulation wave is converted into a frequency of IF band by using a local oscillator 161 and a multiplier 162. This IF signal is converted into a base band signal of in-phase channel by an output signal of a local oscillator 163 and a multiplier 164, and is further converted into a base band signal of an orthogonal channel by the local oscillator 163, a .pi./2 (half-pi) phase shifter 165, and a multiplier 166. The base band signal is suppressed of its high frequency by low-pass filters 167, 168, and is converted into a digital signal by A/D converters 169, 170, and the waveform is shaped by base band filters 171, 172, and the signal is demodulated into a VSB signal by an adder 173.
In such conventional VSB demodulator, since the signal is processed by the analog circuit using the half-pi phase shifter, it is required to compensate for factors of deterioration of orthogonality of the half-pi phase shifter or temperature characteristic by the analog circuit.